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Mohamed G. Gouda						Homework 3
CS 395T: Network Protocol Security				Fall 2006
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1. (3 points) 
Each security association in IPsec is uniquely identifiable in the Internet
by the value triple:
			(SA#, type, dst)
where 	SA# is a 4-byte id given to the security association,
 	type is the type of the security association, AH or ESP, and
	dst is the IP address of the destination of the security association.
What are the advantages of this design decision over making each security
association uniquely identifiable in the Internet by each of the following 
value pairs:
			(SA#, type),
			(type, dst), and
			(SA#, dst)
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2. (4 points)
Specify the rules of a firewall for a private network whose computers have the
IP addresses 128.83.0.0/16. the firewall has two interfaces: interface 0 that
connects the firewall with the rest of the Internet, and interface 1 that
connects the firewall with the private network. The private network has an 
email server whose IP address is 128.83.5.3. This email server accepts email
messages, from the rest of the Internet and from the private network, over
tcp port 25. The firewall does the following:

i. It discards any message intended to establish a tcp connection from a 
computer in the rest of the Internet to any tcp port in any computer in the
private network, other than tcp port 25 in computer 128.83.5.3.

ii. It discards any message from the rest of the Internet if the IP address 
of the source of the message is in 128.83.0.0/16.

iii. It accepts all other messages from the rest of the Internet, other than 
those specified in i and ii.

iv. It discards any message from the private network iff the IP address of the
source of the message is in 128.83.0.0/16.
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3. (3 points)				
A voting system consists of human voters, a certifying server, a voting 
server, and a mix server. The communications between human voters and the 
certifying server are over SSL connections. Also the communication between
the certifying server and the voting server is over SSL. The communications
between human voters and the voting server are via the mix server.   

The certifying server maintains a database that contains a pair of data items 
concerning each voter, namely the voter's name and the voter's voting number.
First, a voter v sends an initiation message, that contains its name and its
voting number, to the certifying server. If the certifying server checks that 
the received pair (voter's name, voter's voting number) is in its database,
then the certifying server records that this voter has already be certified, 
and selects a random nonce n and sends one copy of n to the voting server and
sends another copy of n to voter v. (The certifying server needs to ensure
that the voting server has received its copy of n before voter v has received 
its copy of n.)

Later after voter v receives nonce n from the certifying server, v sends a
message B.mix<B.voting<n, myvote>> to the mix server, where B.mix is the 
public key of the mix server and B.voting is the public key of the voting 
server. The mix server forwards this message, as B.voting<n, myvote>, to the
voting server. The voting server keeps a record of all the nonces that it has
received from the certifying server. When the voting server receives the
message B.voting<n, myvote>, it checks whether n is in its record of all 
nonces received from the certifying server. If so, then the voting server 
records that nonce n has already been used and enters myvote in its vote 
tally. If not, then the voting server discards myvote.

Specify three security properties that this voting system satisfies. Also
specify one security property that this voting system does not satisfy.
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